This invention generally relates to a method for controlling leakage between the primary and secondary sides of a heat exchanger, and is specifically concerned with a method for controlling leakage through degraded sections of heat exchanger tubes in the tubesheet region of a nuclear steam generator.
Systems and methods for eliminating or reducing leakage in the heat exchanger tubes of a nuclear steam generator are known in the prior art. In these systems, a tubular sleeve is inserted in the open end of the heat exchanger tubes in the tubesheet region of the generator. These sleeves are long enough to completely extend across the section of these tubes which may be degraded by corrosion and cracking. When the ends of the sleeve are connected to the inner walls of the heat exchanger tube by either welding or by expanding the sleeve ends into engagement with the inner walls of the tube, the sleeve forms a "hydraulic bridge" across the corroded and cracked section of the tube. The installed sleeve conducts hot, radioactive water from the primary side of the generator through the interior of the heat exchanger tube while preventing this water from seeping through the cracks and corrosion present in the degraded section of the tube and contaminating the non-radioactive water in the secondary side of the tube.
While such sleeving systems and methods have proven to be highly effective in eliminating or at least reducing leakage between the hot, radioactive water in the primary side and the non-radioactive water in the secondary side of the generator, such sleeving techniques require substantial amounts of time and labor, and hence are expensive. However, before the magnitude of these expenses can be appreciated, some general background as to the structure, operation and maintenance of nuclear steam generators is necessary.
Nuclear steam generators are comprised of three principal parts, including a secondary side, a tubesheet, and a primary side which circulates water heated from a nuclear reactor. The tubesheet is disposed between the primary and secondary sides, and serves to hydraulically isolate them from one another. The secondary side of the generator is provided with a plurality of heat exchanger tubes, which may be U-shaped, as well as an inlet for admitting a flow of water. Both the inlet and outlet ends of the U-shaped tubes are mounted in bores in the tubesheet. Such mounting is accomplished by welding the bottom end of the heat exchanger tubes around the bottom edge of the bore in the tubesheet through which it extends.
Hot, radioactive water flowing from the nuclear reactor is admitted into the section of the primary side containing all of the inlet ends of the U-shaped tubes. This hot, radioactive water flows through these inlets, up through the tubesheet, and circulates around the U-shaped tubes which extend within the secondary side of the generator. This water from the nuclear reactor transfers its heat through the walls of the U-shaped tubes to the non-radioactive feed water flowing through the secondary side of the generator, thus boiling the feed water into non-radioactive steam that in turn powers the turbines of an electric generator. After the water from the reactor circulates through the U-shaped tubes, it flows back through the tubesheet, through the outlets of the U-shaped tubes, and into the outlet section of the primary side, where it is circulated back to the nuclear reactor for re-heating.
The walls of the heat exchanger tubes of such nuclear steam generators can suffer from a number of different forms of corrosion degradation, one of the most common of which is intergranular attack (IGA) including cracking. Empirical studies have shown that the heat exchanger tubes are more susceptible to IGA in the crevice regions of the steam generator, the largest being the tubesheet region of the generator for the unexpended-tube design. In this tubesheet design, the heat exchanger tubes are surrounded along their lengths by an annular crevice created by the space between the outer wall of the heat exchanger tube and the surface of the surrounding bore in the tubesheet through which the tube extends. The relatively poor hydraulic circulation in such annular crevices can subject the water that fills these crevices to what is known in the art as nucleate boiling wherein the water is continuously flashed into steam. After long periods of time, such nucleate boiling can reduce various chemical compounds out of the water. These compounds ultimately form a sludge in the crevice. The aggressive chemical agents within these sludges, coupled with the hot spots that form around the heat exchanger tubes in these regions due to the poor or nonexistent hydraulic circulation around them, can promote corrosion on the outer surface of these tubes in the tubesheet region. Such corrosion can ultimately cause the tube walls to crack, and thereby set the stage for a leak of radioactive water from the primary side into the nonradioactive water present secondary side of the generator, thereby radioactively contaminating the steam produced by the steam generator.
In order to prevent such leakage from occurring between the primary and secondary sides, a tubular sleeve may be inserted into the open end of a heat exchanger tube having a degraded section in the tubesheet region. The sleeve is dimensioned that it is long enough to completely extend across the degraded section of the tube. After the sleeve has been so positioned, the bottom and top ends of the sleeve are secured into leak obstructing engagement with the inner walls of the heat exchanger tube by either brazing, welding, or radially expanding the ends of the sleeve with either a hydraulic expander tool, or a roller-type expander tool. Once installed, the sleeve forms a hydraulic bridge across the degraded section of the heat exchanger tube that obstructs water from the primary side of the generator from leaking through the tube walls and into the annular crevice between the tubesheet bore in the outer surface of the heat exchanger tube and from thence into the secondary side of the generator.
Unfortunately, such sleeving techniques require large amounts of time and effort as a single nuclear steam generator may have over 4,000 heat exchanger tubes mounted within its tubesheet. The financial losses become even more apparent when one considers that the steam generator must be completely shut down before any sleeving operation may be implemented, and that each day of shutdown typically costs the utility owners over $500,000 per day in lost revenues. The installation of such sleeves also reduces the available inner diameter of the tube which not only increases the flow resistance within the tube, but also makes it very difficult to slide other types of maintenance devices through the tube, such as the radiant heater probes used by the Westinghouse Electric Corporation to heat treat its U-bends of such tubes. Finally, such sleeving operations can induce residual tensile stresses in the tube walls which can promote additional corrosion, thus partially defeating the purpose of the sleeving operation.
Clearly, there is a need for a method for stopping or at least controlling leakage through the walls of degraded heat exchanger tubes in the tubesheet region of a nuclear steam generator that is faster and easier to implement than prior art sleeving techniques. It would be desirable if such a method did not result in the reduction of the inner diameter of the tube, and did not impart any substantial tensile stresses in the walls of the tube.